Conventionally, as one exhaust purification device that purifies NOx in exhaust, a device has been proposed in which a selective reduction catalyst that selectively reduces NOx in the exhaust by adding a reducing agent is provided in an exhaust channel. For example, a selective reduction catalyst of urea addition type that uses urea water as a reducing agent generates ammonia from the urea thus added, and selectively reduces NOx in the exhaust with this ammonia.
With such a selective reduction catalyst, in a case in which the injection amount of reducing agent is less than the optimum amount, the NOx reduction rate declines from the ammonia consumed in the reduction of NOx being deficient, and in a case of being greater than this optimum amount, the ammonia that is surplus in the reduction of NOx is discharged. As a result, with exhaust purification devices equipped with a selective reduction catalyst, it has been important to appropriately control the injection amount of the reducing agent. Consequently, devices that estimate the NOx reduction rate of the selective reduction catalyst and control the injection amount of the reducing agent based on this estimation are exemplified in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2006-274986) and Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2004-100700).
The exhaust purification device of Patent Document 1 detects the NOx concentration on a downstream side of the selective reduction catalyst, and estimates the composition of exhaust flowing into the selective reduction catalyst, i.e. the ratio of NO to NO2, from this detected NOx concentration and the operating state of the internal combustion engine. Furthermore, it estimates the NOx reduction rate of the selective reduction catalyst based on the composition of this exhaust, and controls the injection amount of the reducing agent.
In addition, the exhaust purification device of Patent Document 2 detects the temperature of the catalyst as an amount related to the NOx reduction rate of the selective reduction catalyst, and controls the injection amount of the reducing agent based on this temperature.
However, the NOx reduction rate of the selective reduction catalysts change not only by the aforementioned such composition of the exhaust and temperature of the selective reduction catalyst, but also according to the degradation state of the selective reduction catalyst. In addition, there is variation in purification performance between individual catalysts. In addition to this, in a case of ammonia being stored in the selective reduction catalyst, the NOx reduction rate of the selective reduction catalyst will apparently change due to the optimum amount of reducing agent differing. Therefore, it is difficult to always optimally control the injection amount of reducing agent with exhaust purification devices such as those exemplified in Patent Documents 1 and 2.
Consequently, technology to more directly detect the NOx purification rate of a selective reduction catalyst and to control the injection amount of reducing agent based on this will be considered hereinafter.
FIG. 16 is a schematic diagram showing a configuration of a conventional exhaust purification device 80.
As shown in FIG. 16, in an exhaust channel 82 of an engine 81 are provided, in order from an upstream side to a downstream side, an oxidation catalyst 83, a urea injection value 85 that injects urea water as a reducing agent, which is stored in a urea tank 84, into the exhaust channel 82, and a selective reduction catalyst 86 that reduces NOx in the exhaust under the presence of urea water. In addition, as a device that monitors the purification performance of the selective reduction catalyst, a temperature sensor 87 that detects the temperature of the selective reduction catalyst 86 and a NOx sensor that detects the NOx concentration on a downstream side of the selective reduction catalyst 86 are provided.
With this exhaust purification device 80, for example, the NOx concentration of exhaust emitted from the engine 81 is estimated using a map established beforehand, and an injection amount of urea water from the urea injection value 85 is determined based on this NOx concentration and the catalyst temperature detected by the temperature sensor 87. In particular, the degradation state of the selective reduction catalyst 86 can be estimated here based on a difference between the NOx concentration detected by the NOx sensor 88 and the NOx concentration of exhaust estimated. With this exhaust purification device, it is possible to correct the injection amount of urea water according to the degradation state of the selective reduction catalyst 86 estimated in the above way.